1. Field of the Invention
This invention relates generally to techniques for testing soils, and particularly to techniques for testing soils involving the application of disturbances to an object embedded in the soil to evaluate the response of the object and the soil to such loading.
2. Background Art
It is often important to determine, at least by estimate, the resistance of a soil to liquefaction, the degradation characteristics of a soil, the dynamic shear modulus of a soil at low levels of shear deformation and the variation in the dynamic shear modulus of a soil with shear deformation. Liquefaction is the total loss of the stiffness and strength of a saturated soil, caused by increased pore water pressure which can result from cyclic loading. Degradation is the reduction in stiffness also due to the buildup of pore water pressure caused by cyclic loading. Degradation may or may not lead to liquefaction depending on the type and state of the soil. The shear modulus is basically the shearing stiffness of a soil. Generally, the shear modulus of a soil is a function of shearing deformation. For example, most soils show reduced stiffness with increasing deformation under monotonically increasing loading.
Commonly these properties are necessary for analyses which predict the response of a site or foundation-structure system to dynamic loading caused by earthquakes, ocean waves or mechanical vibrations. Conventionally, these properties have been determined by conducting laboratory tests on samples recovered from a site or by in situ field tests.
Laboratory testing of soil samples suffers from a number of problems. Particularly, the acts of recovering a sample, transporting it to a laboratory and preparing the sample for a test, can so disturb a sample from its original state, as to bring to question any results obtained. In addition, it is often difficult to reproduce the original field environment (state of stress) of the sample because it is often difficult and costly to define the environment and because typical laboratory test apparatus are limited in their ability to reproduce environmental conditions. Because of the failure to precisely account for environmental considerations, laboratory tests are subject to error for this additional reason. Safely accounting for these disturbances and for environmental conditions may lead to excessively costly structures.
The field testing of soils also suffers from a number of problems. Liquefaction resistance is generally tested in the field by a penetration test. Conventionally, a closed ended probe is either penetrated into the ground at a controlled slow rate, simulating static, non-cyclic loading but introducing severe failure into the local soil, or a cylinder is driven into the ground by violent impacts, also causing severe and immediate failure in the soil local to the cylinder. The resistance of the soil to liquefaction is correlated to the resistance of the probe or cylinder to penetration. Neither of these tests induces the type of loading generally induced by earthquakes or ocean waves which are the main known causes of liquefaction.
Generally, earthquakes and ocean waves generate a lower amplitude loading which does not produce the magnitude of stresses needed for severe, immediate failure. Rather, the soil is excited at a lower amplitude of stress for a number of cycles. Generally, each cycle causes the soil to degrade incrementally and liquefaction is achieved only after a number of cycles. Hence, the phenomena induced by penetration tests are different from those of real interest, bringing into question the validity of the correlation of liquefaction resistance to penetration resistance.
The fact that the desired loadings are not reproduced in penetration tests leads to other problems. For example, a number of common factors, such as age, state of stress, stress history, and the like, affect significantly liquefaction resistance as well as the resistance of an object to penetration. However, it is unlikely that these factors affect liquefaction resistance to the same degree that they affect the resistance to penetration. This brings into further question the validity of a correlation between liquefaction resistance and penetration resistance. As a result of such uncertainties, widely used correlations between liquefaction and penetration resistance are deliberately very conservative and can lead to costly designs for major structures.
In addition, correlations are not available for all of the different types of soils which may be prone to liquefaction. Thus, there is even a greater uncertainty in estimating liquefaction resistance from penetration test results for a site consisting of soils with no significant testing history. A further drawback to in situ penetration testing is that this type of testing does not readily provide the type of information needed to conduct the refined analyses which are often necessary for site and foundation response studies.
While in situ testing procedures have not been widely used to obtain degradation characteristics, a number of in situ tests have been used to determine the dynamic shear modulus and to a lesser extent, its variation with shear deformation. These include wave propagation tests, resonant footing tests and downhole probe tests. There are several different wave propagation techniques. With these techniques, the shear modulus of the soil is estimated from the measurement of some wave parameter, such as wave speed or wavelength. Each of these techniques has limitations or drawbacks. One technique, known as "seismic crosshole testing", requires two or more bore holes with sensors, and a below ground excitation source, making it relatively expensive for testing in a normal environment and difficult to practice in an offshore environment. A second technique, known as "seismic downhole testing", requires only one bore hole but is limited to measurements involving very low strain amplitude. A third technique, known as "seismic refraction", can result in poor definition of layering for sites where interbedded layers exist. A fourth technique, involving surface wave generation, requires sizeable equipment to provide definition of layering to the depths typically of interest.
In resonant footing tests for obtaining dynamic shear modulus, a footing located at the surface is vibrated to determine its resonant frequency. With this procedure, only the near surface shear modulus can be estimated. It is usually desirable, however, to also obtain the below surface characteristics.
There are several downhole probes for measuring shear modulus. One probe measures the shear modulus of the walls of a bore hole. The material along the bore hole wall can be very disturbed due to the drilling activity, and may give results that are not representative of undisturbed soil. It is believed that with this technique, difficulties would be experienced in taking measurements in a cased bore hole. A second probe, disclosed in U.S. Pat. No. 3,643,498 to Hardin, has similar capabilities and potential problems. Additionally, this probe may be penetrated below the base of the bore hole but this device probably would displace a considerable amount of the soil in the immediate vicinity of the measurement. Thus, the zone of soil having the greatest influence on measurements would probably be highly disturbed and therefore, to some degree, unrepresentative of the undisturbed soil.
While the state of the technology in this field has experienced rapid advancement, and many of the techniques now known have important advantages, the inventor of the present invention has identified certain characteristics that would be particularly advantageous if implemented in a single device. These characteristics include minimal soil disturbance by the testing probe, preservation of the original environment of the test soil, in situ testing using loading comparable to that experienced during the real life phenomena that induce soil failure, and the capability of providing liquefaction resistance, degradation characteristics, shear modulus, and the variation in shear modulus with shear deformation. Further, it would be advantageous if such a device could readily enable the quantification of natural phenomena such as liquefaction and degradation.